If you had the ability to control the time, you could prove this by stopping the motion of flowing sand in an hourglass. At any time during your performance you can start and stop the sands of time at your command. Only the magician can control the time to stop and start again. After the performance, you can hand out the hourglass to be examined. This is REAL magic!

You can also make a die (or coin, etc.) penetrate the hourglass visually and cleanly at any time during your performance. After the performance, you can hand out the hourglass to be examined.